The geology and genesis of the Viburnum Trend Pb-Zn-Cu-(Ag) Mississippi Valley–type ore district, USA

Abstract

ABSTRACT The Viburnum Trend, a Mississippi Valley–type Pb-Zn-Cu-(Ag) ore district, USA, is primarily hosted within the Cambrian Bonneterre Formation. Primary ore mineralization consists of galena, sphalerite, and chalcopyrite. Primary gangue minerals include dolomite and calcite, with lesser amounts of quartz, pyrite, and marcasite also present. The paragenetic sequence is complex, involving multiple generations of sulfide ore minerals. Early minerals are disseminated throughout the host rock. The main ore-stage deposition mostly occurs as colloform ore, while late-stage deposition primarily includes vug-filling ore. The geochemistry of deposits within the trend requires a complex fluid history, consistent with the mixing of multiple chemically distinct fluids to initiate ore precipitation. Salinities of included fluids range from less than 5 to ~30 wt% equivalent NaCl. Fluid chemistries align with the presence of a northward-migrating fluid, likely propelled through the Arkoma Basin due to hydrologic head provided by the Ouachita orogen, and a southward- or westward-migrating fluid, likely escaping the Illinois Basin due to overpressuring during sediment compaction and associated subsidence. Ore deposition probably commenced through fluid mixing. Lead and sulfur isotope chemistries of included fluids and associated solid phases suggest that multiple sources contributed lead and sulfur present in ore-stage mineralization. Additionally, ore fluids seem to have interacted with igneous basement rocks, likely acquiring lead, copper, and sulfur during migration through faults and fractures. The Viburnum Trend is relatively well understood; however, significant questions persist regarding fluid flow–driving mechanisms and pathways, sulfur sources, sulfate reduction reactions, metal sources, interaction with basement rocks, and the role of organic material in the host rock.